NEW YORK (Reuters Health) - Americans of Indian descent may have a heightened risk of diabetes, even when their weight is in the normal range, research suggests.

Researchers found that among more than 7,400 Asian Americans in a national health study, those of Indian descent had roughly three times the risk of diabetes as U.S. whites as a whole, and a higher prevalence of the disease than people of other Asian ethnicities.

Moreover, although type 2 diabetes is closely associated with obesity, Asian Indians' risk began to climb even at lower weights.

Among those with weights in the normal range -- as measured by body mass index (BMI), a ratio of weight in relation to height -- about 7 percent had diabetes. That compared with rates of anywhere from 2 percent to 4 percent among whites and people of other Asian ethnicities.

Among obese adults -- as defined as a BMI of 30 or higher -- one-third of Indian adults had diabetes. Among other ethnicities, the rate ranged from 12 percent to 17 percent.

The reasons for the findings are not entirely clear, Dr. Reena Oza-Frank and colleagues at Emory University in Atlanta point out in the journal Diabetes Care.

However, the researchers note, ethnic differences in body fat levels and body fat distribution may play a role.

Studies have shown that compared with people of other races and ethnicities, Asians tend to have a higher proportion of body fat at any given weight -- along with a higher type 2 diabetes risk.

It's possible that people of Indian background are particularly likely to have body fat distributions -- including more abdominal fat -- that are closely associated with diabetes, according to Oza-Frank and colleagues.

Some studies, the researchers note, have suggested that this may be a factor in the higher diabetes risk seen in Indian adults compared with Europeans.

Because Asian adults tend to have more body fat -- and, therefore, greater health risks at relatively lower weights -- the World Health Organization (WHO) has proposed lowering the threshold for how "overweight" is defined in Asian populations.

The standard definition of overweight is a BMI of between 25 and 30; the WHO recommendation sets that at 23 to 27.5 for Asians.

However, the current findings, Oza-Frank and colleagues write, suggest that for Asian Indians, "ethnicity alone may be as informative as BMI with regard to diabetes risk."

They call for more research into the "complex relationships" among body size, body fat and disease risks across different Asian populations.